1. Field of the Invention
The present invention relates to a printing apparatus with heating unit for heating printed print media.
2. Description of the Related Art
In image formation based on ink jet printing, an image is formed on a print medium by ejecting ink onto the print medium. When printing is performed according to such an ink jet method at high speed, immediately after a printed print medium is placed at a sheet discharging position, the next printed print medium may reach the sheet discharging position. This precludes a sufficient time to dry the print medium and to provide dry ink on the print medium. Thus, a printed image portion of the first printed print medium may come into contact with the second printed print medium. Furthermore, when a printed print medium is laid on top of a preceding printed print medium immediately after the preceding printed print medium has been discharged, ink may adhere to the back side of the printed print medium laid on the preceding printed print medium. As a result, the quality of printed matter may be degraded. Thus, to dry and fix the ink in a short time, a printing apparatus is used which includes a mechanism for heating and drying printed print media.
Japanese Patent Laid-Open No. 2003-326680 discloses a fixing device with a preheater and a heater. In the fixing device, each of the preheater and the heater is divided into a plurality of heaters so that heaters corresponding to areas of a printed print medium with images formed therein are activated based on image information in the printed print medium. This procedure reduces the consumption of energy in the fixing device for fixing images and drying print media.
In general, a heating resistor, a halogen heater, a dielectric heating system, or the like is adopted as a heating source for heating printed print media. However, if such a heating source is used, for example, Joule heat generated by current is utilized. However, a conversion of current into heat is likely to result in a loss. Hence, in general, electric energy generated by current is not very efficiently converted into heat energy, leading to a relatively low heat efficiency of the heating source.
Thus, the fixing device for heating and fixing printed images may adopt a thermal conversion device that utilizes a vapor-compression refrigeration cycle with a heat-pump function. The use of such a fixing device allows heat recovered from surroundings to be supplied to a refrigerant, which concurrently functions as a heating source. The thermal conversion efficiency of the fixing device is thus improved. This reduces the energy consumed when printed print media are heated.
If a heat pump is utilized in a heating and drying device for print media as a heating source, heat dissipated during a refrigerant-condensation process is generally utilized to heat the print media. However, when a condenser is used to carryout thermal conversion between the refrigerant and a print medium to be heated, even if the refrigerant is superheated steam with a temperature higher than the condensation temperature, heat is exchanged between the refrigerant and the print medium to immediately lower the temperature of the refrigerant down to the condensation temperature. Thus, when the print medium is heated during the refrigerant-condensation process, once the temperature of the refrigerant reaches the condensation temperature, the print medium and the refrigerant have an equal temperature. This precludes the temperature of the print medium from being further increased. As described above, the heat of the superheated steam cannot be effectively used, and thermal efficiency is thus insufficient. This leads to extra driving of a compressor and the possible consumption of extra power.
Furthermore, the print medium cannot obtain a sufficient amount of heat dissipated from the condenser. Thus, the maintenance of such a state for a long time results in insufficient heat dissipation from the condenser. As a result, the refrigerant is decompressed in the condenser without being supercooled. This prevents a stable refrigeration cycle from being maintained, and increases the operating pressure of the compressor. The operating pressure and power consumption of the compressor are in an almost proportional relationship, and the heat pump thus consumes increased power.